A series of experiments are proposed to determine the precise nature of the structural alteration of the luminal membrane of epithelial cells responding to vasopressin. There now appear to be several components of the membrane that are altered by the hormone: an aqueous pathway, a facilitated transport system for urea and other amides, a sodium pathway, and an additional pathway for uric acid and drugs. Using techniques of membrane isolation, tests of co-and counter-transport, and surveys of renal handling of urea in many species, we plan to clarify the nature of the facilitated transport system for urea, and the important implications of the new drug pathway.